This invention relates to electric lamps and, more specifically, to low pressure (e.g. fluorescent lamps) operated at low and intermediate pressures at frequencies from 50 kHz to 3 MHz.
Electrodeless fluorescent lamps utilizing an inductively coupled plasma have been widely used for indoor and outdoor applications. These lamps have longer life than conventional fluorescent lamps employing heating filaments. Presently, however, only a few electrodeless lamps have been brought to market. Most of them have a bulbous envelope: xe2x80x9cGenuraxe2x80x9d (GEC), QL (Phillips), xe2x80x9cEverlightxe2x80x9d (MEW). Few are used for general lighting. They are not suitable for applications where long lamps with axially uniform light output are required (e.g. tunnel lighting).
A closed-loop electrodeless fluorescent lamp operated at a frequency of 250 kHz was recently introduced on the market by Osram/Sylvania and described in U.S. Pat. No. 5,834,905 by Godyak et al. This lamp has uniform light output along the envelope of 400 mm length and can be used in tunnel lighting. However, the width of that lamp is a rather large (140 mm) to fit in many reflectors used in tunnel lighting fixtures.
U.S. Pat. No. 5,382,879 to Council et al. described a long tubular fluorescent lamp operated at RF frequency from 30 MHz and higher. UV and visible radiations are produced by capacitive discharge plasmas generated inside the tube with the help of inner or outer RF electrodes positioned on the tube walls. However, the plasma power efficiency of a capacitive discharge operated without magnetic field at RF frequencies of f less than 400 MHz is relatively low since most of the RF power goes for the ion acceleration at the sheath. Also, the cost of the lamp driver at such high frequencies is high.
U.S. Pat. No. 5,760,547 to Borowiec described the electrodeless lamp with a bulbous envelope and a reentry cavity that employs two independently powered induction coils. Such an arrangement causes spreading of the plasma along the axis and results in a more axially uniform light output. However, this lamp is best used for operation at a high frequency (MHz range) where the induction coil of few turns can be used. For efficient operation at lower frequency, f less than 400 kHz, an electrodeless lamp requires low loss ferrite cores. Again, a lamp with a bulbous envelope does not have an axially uniform plasma and, hence, axially uniform radiation as required by the tunnel lighting.
According to the present invention, we have found an efficient electrodeless fluorescent lamp that is suitable for tunnel lighting and is operated at frequencies from 20 kHz to 3 MHz. The lamp comprises a glass, tubular, evacuated envelope having a length between about 50 and 2000 mm and a diameter between about 10 and 500 mm. The lamp further comprises one or more reentry cavities with a ferrite cores disposed in the cavities and a coil wound on each core. The axis of each core is coaxial with the cavity or coplanar with the axis. The cavities have lengths between about 10 and 1950 mm. A thermally conductive cooling rod or tube is disposed in each core and is attached to an external heat sink to draw heat from the cores. When using a tube, the outer diameter is between about 4 and 50 mm and the inner diameter is between about 2 and 50 mm. With a rod, the outer diameter is between about 4 and 50 mm.
A filling of an inert gas and a vaporous metal such as mercury, cadmium, sodium or the like is placed in the envelope. A protective coating is deposited on the vacuum side of the envelope and cavity walls. A conventional phosphor coating is deposited on the protective coating. A reflective coating (alumina or the like) is deposited on the vacuum side of the cavity walls, between the protective and phosphor coatings, to reflect the UV and visible light back to the envelope walls.
Cylindrical cores made from low loss ferrite material (such as ferrous-based MnZn or the like) are positioned inside each reentry cavity. Each core is wrapped with a primary coil which is electrically connected to a conventional matching network. All matching networks are connected in parallel and are powered by a high frequency power source, a driver. The driver generates a voltage at a high frequency, f=20-3,000 kHz, and is connected electrically to a power supply.
An object of the present invention is to design an efficient electrodeless fluorescent lamp suitable for tunnel lighting and operated at a frequency from 20 kHz to 3 MHz and power from 5 W to 1,000 W.
Another object of the present invention is to design an envelope with cavities having the proper position, shape, and size so to provide the sufficient volume inside the envelope for several plasmas needed for the efficient production of the axially uniform visible and UV radiations.
Yet another object of the present invention is to design an assembly that comprises the ferrite core and the induction coil that have very low power losses.
A further object of the present invention is to locate coil/core assemblies in an envelope to avoid the mutual interference of magnetic fields generated by each assembly.
The many other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon reading the following specifications when taken in conjunction with the drawing and claims.